1. Field of the Invention
This invention relates generally to optical communications networks, and, in particular, to a failure tolerant Wavelength-Division Multiplexed (WDM) optical ring communications network.
2. Background of the Invention
It is known to provide protection in ring communication networks against line failures and the like by equipping such networks with bypass equipment for bypassing failed components and routing signals to their intended destinations. For example, some known four-fiber ring networks employ two “working” transmission paths to normally forward communications being exchanged between communication terminals of the network, and two “protection” paths to forward those communications in the event that a failure occurs in one or more of the “working” paths. At least one other prior art ring network employs only two communication paths to forward communications throughout the network during normal operating conditions, and in the event that one of those paths fails, the other path is employed as a backup to forward both its own transmission signals and those of the failed path. Some prior art networks are also equipped with equipment for enabling a “loopback” bypass configuration to be implemented in the event of a path failure, for enabling signals to be forwarded to intended destinations.
Most prior art ring communication networks employ time-multiplexing techniques to time-multiplex channels, and perform switching between channels by rearranging time slots in a predetermined manner. In Wavelength-Division Multiplexed (WDM) networks, on the other hand, optical channel signals having respective wavelengths are multiplexed onto a single waveguide, and are demultiplexed such that each channel signal is individually routed to a predetermined destination. An example of one prior art WDM ring communications network that includes protection equipment is depicted in FIG. 1a, and is identified by reference numeral 11.
The network 11 of FIG. 1a includes optical add/drop multiplexer/demultiplexers (OADMS) 1, 3, and 5, a plurality of communication nodes (also referred to as terminals) 9a, 9b, and 9c, a plurality of switching modules 2, 6, and 4, a plurality of working communication links 7a-7a″, 7b-7b″, and a plurality of protection communication links 8a-8a″, 8b-8b″. The switching modules 2 and 6 are coupled together through the links 7a′, 7b′, 8a′, and 8b′, the switching modules 4 and 6 are coupled together through links 7a″, 7b″, 8a″, and 8b″, and switching modules 2 and 4 are coupled together through links 7a, 7b, 8a, and 8b. 
Referring also to FIG. 1b, a block diagram is shown of an OADM 1′ and a switching module (SM) that is coupled to the OADM 1′. The OADM 1′ of FIG. 1b represents individual ones of the OADMs 1, 3, and 5 of FIG. 1a, and the switching module (SM) of FIG. 1b represents in further detail individual ones of the modules 2, 4, and 6 of FIG. 1a. The switching module (SM) includes a 6×6 optical switch (S) that is coupled to the OADM 1′, and electrical variable optical attenuators (EVOAs) E1 and E2 that are interposed between the OADM 1′ and switch (S). The switch (S) has a plurality of inputs (I), each of which is coupled to a corresponding communication link from FIG. 1a, and also has a plurality of outputs (O), each of which is coupled to a corresponding communication link from FIG. 1a. 
During normal operating conditions (i.e., in cases where the working links 7a-7a″, 7b-7b″ are functioning properly), the switch (S) of each module 2, 4, 6 is maintained in a configuration that enables the terminals 9a-9c to communicate with one another through the OADMs 1, 3, 5, the modules 2, 4, 6, and the working communication links 7a-7b″. However, during cases in which one or more of the working links 7a-7b″ fail(s), then the switches (S) of the modules coupled to those links are configured to enable the failed links to be bypassed, and to enable signals to be exchanged between the terminals 9a-9c by way of selected ones of the protection links 8a-8b″. 
It would be desirable to provide a network having other types of configurations for providing span and/or ring (i.e., loopback) protection against network component failures. It would also be desirable to provide a network in which minimal signal losses are incurred during bypass operations, without requiring a large number of amplifiers and without incurring a narrowing of available bandwidth.